No difference in urinary iodine concentrations between Boston-area breastfed and formula-fed infants

Thyroid hormone is essential for normal mental and physical development in infancy and childhood and is dependent on adequate iodine intake. During the first few months of life, infants are reliant on breastmilk and/or infant formula as their sole sources of dietary iodine. The iodine status of U.S. infants has not been well studied.

This was a cross-sectional study of 95 breastfed and/or formula-fed infants less than 3 months of age in the Boston area. We measured iodine content from infants' single spot urine samples and assessed associations with infant feeding type as well as maternal demographic data, salt and multivitamin use, smoking status, and diet.

The median infant urine iodine concentration was 197.5 μg/L (range 40-897.5 μg/L). Median infant urine iodine concentrations were similar between infants who were exclusively breastfed (n=39, 203.5 μg/L; range 61.5-395.5 μg/L), formula-fed (n=44, 182.5 μg/L; range 40-897.5 μg/L), and mixed (n=10, 197.8 μg/L; range 123-592.5) (p=0.88). There were no significant correlations of infant urinary iodine with maternal salt or multivitamin use (regularly or in the past 24 hours), active or secondhand cigarette smoke exposures, infant weight, infant length, or recent maternal ingestion of common iodine-containing foods, although the correlations with iodine-containing foods are difficult to accurately determine due to the small sample sizes of these variables.

Both breastfed and formula-fed infants less than 3 months of age in the Boston area were generally iodine sufficient. Larger studies are needed to confirm these observations among infants nationwide and elucidate other factors that may contribute to infant iodine nutrition.